NEW ORLEANS | The federal government gave BP the green light Wednesday to try choking off the Gulf of Mexico oil gusher with an untested metal cap after a daylong delay to satisfy worries about whether the project might make the leak worse.

National Incident Commander Thad Allen said at a news briefing that testing would begin later Wednesday after government scientists carefully reviewed whether new leaks would erupt from the well once it was sealed off.

“There is a tremendous sense of urgency,” he said, but added that nobody wants to make “an irreversible mistake.”

BP had zipped through weekend preparations and gotten the 75-ton cap in place Monday atop the well. The device is meant is to stop the oil and pump excess to ships, raising hopes the gusher could be checked. BP was getting ready to test pressure on the well by closing valves in the cap when the government intervened late Tuesday.

The sudden delay was another jolt for Gulf residents already bruised by a string of BP failures to stop the oil from ruining tourism and fishing, the coast’s two biggest industries.

Mr. Allen said the delay was necessary to settle lingering questions about whether the cap, once the valves are closed, could force oil under pressure to create new leaks.

“We sat long and hard about delaying the tests,” Mr. Allen said. But he said that in the interest of the public, the environment and safety, the pause was necessary, and now they were convinced the test can go forward.

“This has been a substantial impact on our environment, this has been a substantial impact on the Gulf Coast, the people, the culture. What we didn’t want to do is compound that problem by making an irreversible mistake,” Mr. Allen said.

The 24-hour holdup involved intense scrutiny of data and consultation with outside experts, Mr. Allen said. One of the most important factors in favor of the testing was a seismic survey that showed the wellbore, which lies below the seafloor, was free of leaks, Mr. Allen said.

One of the concerns federal officials had about the testing was the possibility of leaks below the ocean floor, but Mr. Allen said the survey “removed the possibility of a negative event” such as gas and oil escaping through cracks in the wellbore.

Such a situation would make the planned permanent fix of pumping heavy drilling mud and cement into the well from below immeasurably more difficult.

Another seismic survey will be done at the end of the 48-hour testing period, a time frame during which the oil should be completely choked off from the Gulf.

The test involves slowly closing the valves of the cap, ultimately blocking the flow of oil entirely. High pressure is a good sign, because it means there’s a single leak.

Mr. Allen said BP will monitor the results of the gradual test every six hours and end it after 48 hours to evaluate the results.